A conventional system for performing authentication between AV units will be described below by referring to FIGS. 2 and 3.
First, in FIG. 2, a digital AV data transmitting unit STB 18 is provided with a public key and secret key 20, authenticating means 19, a digital interface D-I/F 22, and enciphering means 19 (sic). The public key and secret key 20 is connected with the digital interface D-I/F22 through the authenticating means 19. Moreover, the enciphering means 19 (sic) is able to refer to the public key and secret key 20 and connects with the digital interface 22. A digital AV data receiving unit TV 23 is also provided with a public key and secret key 26, authenticating means 25, a digital interface D-I/F 24, and decoding means 27. The public key and secret key 26 connects with the digital interface D-I/F 24 through the authenticating means 25. Moreover, the decoding means 27 is able to refer to the public key and secret key 26 and connects with the digital interface D-I/F 24. Furthermore, the digital interface D-I/F 22 and the digital interface D-I/F 24 are constituted so as to be able to transfer data each other.
Then, operations between the digital AV data transmitting unit STB 18 and the digital AV data receiving unit TV 23 are described below. First, the digital AV data receiving unit TV 23 outputs an authentication request. Then, the authentication request reaches the digital interface D-I/F 22 constituting the digital AV data transmitting unit STB 18 through the digital interface D-I/F 24. The digital interface D-I/F 22 receives and authenticates the authentication request by referring to the public key and secret key 20 by the authenticating means 19. When the public key and secret key 20 is authenticated by the digital AV data transmitting unit STB 18, data is enciphered by the enciphering means 21 and enciphered data is transmitted through the digital interface D-I/F 22. The data is decoded by the decoding means 27 through the digital interface D-I/F 24 by referring to the public key and secret key 26.
Thus, a function strong against forgery and alteration can be realized. However, authentication using public key and secret key requires a lot of time. In case of the data not very important such as news, unnecessary time may be required for authentication. Moreover, as for a unit for receiving only data which can be copied such as a VTR, a digital AV data receiving unit may not require a strict authentication according to circumstances. In this case, time is wasted.
Then, in FIG. 3, a digital AV data transmitting unit STB 28 is provided with a common key 30, authenticating means 29, a digital interface D-I/F 32, and enciphering means 31. The common key 30 connects with the digital interface D-I/F 32 through the authenticating means 29. Moreover, the enciphering means 31 is able to refer to the common key 30 and connects with the digital interface 32. A digital AV data receiving unit TV 33 is also provided with a common key 36, authenticating means 35, a digital interface 34, and decoding means 37. The common key 36 connects with the digital interface 34 through the authenticating means 35. Moreover, the decoding means 37 is able to refer to the common key 36 and connects with the digital interface 34. Furthermore, the digital interface 32 and the digital interface 34 are constituted so as to be able to transfer data each other.
Then, operations between the digital AV data transmitting unit STB 28 and the digital AV data receiving unit TV 33 are described below. First, the digital AV data receiving unit TV 33 outputs an authentication request. Then, the authentication request reaches the digital interface D-I/F 32 constituting the digital AV data transmitting unit STB 28 through the digital interface D-I/F 34. The digital interface D-I/F 32 receives and authenticates the authentication request by referring to the common key 30 by the authenticating means 29. When the request is authenticated by the digital AV data transmitting unit STB 28, data is enciphered by the enciphering means 31 and enciphered data is transmitted through the digital interface D-I/F 32. The data is decoded by the digital decoding means 37 through the digital interface D-I/F 34 by referring to the common key 36.
Thus, it is possible to authenticate data in a short time. However, authentication using a common key is weak in forgery and alteration. Therefore, in case of the data important for a copyright such as a new movie, data may be looked and listened by a third party free of charge. Moreover, it is necessary to correspond to a case of connecting with a unit for performing a strict authentication in order to display all received data such as a case of TV or there is a case in which a digital AV data receiving unit requires a strict authentication. In this case, a trouble may occur that the copyright of important data is not protected.
Therefore, there are problems that a lot of time is required to authenticate the data not very important and that the authentication of important data is weak in forgery and alteration. Moreover, there are some digital AV data receiving units that do not require a strict authentication. When applying a strict authentication to these units, there is a problem that time is wasted. However, there some digital AV data receiving units which require a strict authentication. When applying a loose authentication to these units, there is a problem that the copyright is not protected. Moreover, when preparing an enciphering key for a strict authentication and a loose authentication respectively in order to prevent illegal use, it is necessary to newly perform a loose authentication even if loose data is necessary after performing a strict authentication and obtaining an enciphering key. Furthermore, when a receiving side uses a unit not having a function for excluding units, there is a problem that a transmitting side is constituted so as not to be able to exclude illegal units.